Potential GOP presidential candidates who gathered here Saturday for an agriculture summit largely agreed on issues such as environmental policy, their objections to the labeling of genetically modified food and what they termed the overreach of the federal government.

But the Renewable Fuel Standard, which is of special interest to the Iowa voters who hold the first presidential nominating contest in the nation, was an exception.

The fuel standard calls for 10% of the nation's fuel to come from renewable sources. The mandate benefits Iowa's corn farmers, who produce the base ingredient of ethanol. Supporters say the standard has created more than 70,000 jobs in Iowa.

New Jersey Gov. Chris Christie said he “absolutely” supported the standard, while Texas Sen. Ted Cruz and former Texas Gov. Rick Perry said they opposed it as a form of government intervention in the marketplace.

"I just fundamentally have a philosophical disagreement that Washington, D.C., needs to be empowered," said Perry, who connected with the crowd by recounting growing up on a farm, showing calves at 4-H fairs, and serving as Texas' agriculture commissioner.

Former Penn. Sen. Rick Santorum said he supported the standard because it reduces reliance on foreign oil.

"Liquid fuel historically has been a big problem for us in this country from a national security perspective," he said.

Former Florida Gov. Jeb Bush tried to take a middle road on the standard, saying the mandate was necessary in the past but the industry’s success meant it was not necessary in the future.

The fuel standard is today what ethanol subsidies once were in Iowa — a key test for candidates.

Opposition to the standard could come back to haunt candidates if they decide to run for the White House in 2016. Gov. Terry Branstad kicked off the forum Saturday morning by telling the crowd that his first advice to presidential candidates is “Don’t mess with RFS!”

The forum was the creation of prominent Iowa GOP donor Bruce Rastetter, who made his money in ethanol and pork operations. He said in an interview that he wanted to get the candidates talking about agricultural issues because he felt they had been pushed to the sidelines during recent presidential caucuses.

"The goal is to get potential candidates to actually think about agricultural issues that affect all Americans," he said in an interview. "It's important to every American and the reason is every American eats. Every American is concerned about food safety, the environment, conservation."